The reaction of fatty acid chlorides with tertiary amines in a variety of inert solvents to make alkyl ketene dimers, which are useful as constituents of paper sizes as well as for other applications in industry, is well known, for instance from U.S. Pat. No. 2,238,826 and an article by J. C. Sauer, Ketene Dimers from Acid Halides, in the Journal of the American Chemical Society, 69 2444-8 (1947).
That reaction in the inert solvent produces alkyl ketene dimer (AKD) in solution and tertiary amine hydrochloride as a finely divided precipitate. Conventionally, the precipitate is separated from the dimerization slurry by liquid/liquid aqueous extraction of the amine hydrochloride salt. The filtrate containing the alkyl ketene dimer and excess tertiary amine is then stripped of solvent under reduced pressure using a rotary evaporator and a water bath at about 80.degree. C. to give the alkyl ketene dimer. The product typically has an alkyl ketene dimer assay of 83% or higher and a non-volatile content up to 99.8%.
Among the solvents conventionally used are benzene, toluene, xylene, ligroin, chlorobenzene, dichlorobenzene, diethyl ether, dibutyl ether, chloroform, carbon tetrachloride, and trichloroethylene. They can be classified as:
1. alkanes and alkane petroleum fractions; PA1 2. aromatic hydrocarbons such as benzene, toluene, and xylene; PA1 3. chlorinated solvents such as chlorobenzene, dichlorobenzene, chloroform, carbon tetrachloride, propylene dichloride, and trichloroethylene; and PA1 4. ethers such as diethyl ether, diisopropyl ether, diisoamyl ether, and di-n hexyl ether. PA1 1. Esters; including ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, ethyl propionate, and methyl butyrate; PA1 2. Ketones: including methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), and cyclohexanone; and PA1 3. The more polar ethers, such as anisole (methyl phenyl ether).
The conventional dimerization solvents typically used in current industrial applications are either unacceptably toxic, particularly benzene and toluene, or are potentially environmentally objectionable, including the whole class of the chlorinated solvents. Thus the need exists for an alkyl ketene dimerization process that uses a solvent with less potential for health and environmental problems than aromatic hydrocarbons or halogenated solvents.
However, in some of the known solvents the alkyl ketene dimerization reaction can produce a low yield of AKD (for instance, below 80%), an excessively viscous dimerization mixture, or very small amine-hydrohalide crystals in the dimerization slurry. If the dimerization reaction slurry is either highly viscous or contains amine-hydrohalide crystals that are very small, separation of the tertiary amine hydrochloride precipitate from the reaction mixture can be extremely difficult.
Also, the conventional separation of salt precipitate from the dimerization slurry, involving liquid/liquid aqueous extraction of the salt, introduces moisture that is likely to cause hydrolysis of the alkyl ketene dimer and lessen the yield of the product (or to use the more accurate term, the "assay", which refers specifically to the yield of pure product in the gross amount produced).
Because the extraction method reduces the AKD assay, it is desirable to produce the amine-hydrohalide precipitates in such a way that they can readily be separated by such mechanical methods as filtration, sedimentation and decantation, possibly after centrifugation, or a combination of centrifugation and filtration, while at the same time avoiding health and environmental problems.